The present invention is directed to a quantitative dispenser for small amounts of liquid samples or reagents and more specifically to a quantitative dispenser using an optical sensing device for controlling the amounts of liquid to be dispensed.
In the fields of biology and medicine, various methods of analysis have been proposed for detecting trace substances in body liquids and, in correlation with the analyses, various systems and devices have been proposed for automatic qualitative and quantitative analyses. One of the common requisites arising in these systems and devices is the strict control of the amount of liquid samples and reagents to be added to the reaction chamber and the prevention of contamination of the samples from each other. For this reason, pipettes, such as those called micropipettes, are usually employed which are manufactured with a high degree of precision and contain a disposable tip. In employing these micropipettes, the disposable tips are disposed of after each sample to avoid contamination of the samples from each other. The sample is usually sucked into the pipette by means of a negative air pressure.
As mentioned above, the strict control of the amount of the liquid samples in reagents is necessary for minimizing variations in the amounts of the samples and for obtaining reliable results, particularly in an immunological estimation. On the other hand, the use of disposable tips is also desirable to avoid contamination of the samples from each other since the concentration ratio of minute components can sometimes reach up to 10.sup.4 -10.sup.6 depending on samples in the immunological measurements of the biological materials. Even when micropipettes having disposable tips are used it is still absolutely essential that the minute amounts of liquid to be dispensed must be determined with a high degree of precision and that controls are necessary for positioning the micropipettes relative to the reservoir from which the liquid samples or reagents are taken. More particularly, if a negative pressure is utilized for taking up the sample solution, the negative pressure can be strictly controlled when the pipette is inserted in a sample vessel. The depth to which the nozzle of the pipette is immersed in the sample solution is likely to vary depending on the size of the sample. The variation may give rise to an error in the quantitative dispensing which cannot be ignored. The same problem also arises when the surface of the sample solution is concave due to the formation of a meniscus or the vessel itself is in a slant position and the diameter of the vessel is small.
In the past it has been proposed to install detectors to sense the level of the liquid surface in a dispensing device. However, an electrode type of detector involves contamination and a non-contact type optical detector in general is not sufficiently precise with tolerances of several millimeters and furthermore cannot accurately operate with turbid solutions or slanted surfaces. In fact an error of several millimeters in immersion may introduce, for example, up to ten percent of dispersion when 5 .mu.l of solution is taken with a pipette of 200 .mu.l capacity.